The present invention relates to a rear projection screen for use in a projection television set and the like.
As shown in FIG. 1, a rear projector used in a video projection television set is adapted to expand light emerging from a light source (P), e.g. a CRT, by means of a lens system (L), project the light from the rear side of a screen (S), and allow a viewer to observe a picture from the opposite side of the screen (S). However, if the distance from the light source (P) to the screen (S) is prolonged as shown, the projector becomes large size. In practice, therefore, a method is adapted whereby one to three mirrors (M) are combined, and the light is projected after being reflected on them once, as shown in FIGS. 2 (A), 2(B), 2(C). However, there have been drawbacks in that, in the method of FIG. 2(A), the height of the projector becomes large, while, in the methods of FIGS. 2(B) and 2(C), it cannot be said positively that the projectors have been made sufficiently compact in terms of the height and depth.
Many types of screens used for such a projector are provided with a circular Fresnel lens on the incidence side, thereby providing a means for allowing every corner of such screens to be brightly when viewed. However, since lens surfaces (a) are continuously formed via non-lens surfaces (b), as shown in FIG. 3, this circular Fresnel lens has a drawback in that the light incident upon the non-lens surfaces (b) indicated by the oblique lines do not converge, thereby reducing the efficiency of the Fresnel lens and adversely affecting the resolution. To prevent this from occurring, a measure is adopted in some cases to dispose the lens surfaces of the Fresnel lens to receive the light from the flat surface and to combine the Fresnel lens with a lenticular lens. However, such an arrangement of the screen with two lenses has disadvantages since the structure of the screen becomes complicated, the picture fades due to the flare occurring between the two lenses, and the efficiency of utilization of light declines.
In addition, screen apparatus which is capable of reducing the depth of a projector, those which are disclosed in Japanese Patent Unexamined Publication Nos. 57120/1983 and 9649/1984 are known, which propose introducing the light obliquely with respect to a screen, thereby reducing the depth of a projector system. However, since these apparatuses make use of the refraction of a lens, there has been a limit to enlarging the angle of incidence.
To overcome these drawbacks, a rear projection screen has been proposed for observing an image by introducing light at a sharp angle from the rear, characterized in that a plurality of prisms are provided in parallel with each other on the light incidence surface, each having a total reflection surface, so that the incident light is totally reflected on the total reflection surface of the screen and is then emerged from the front viewing surface of the screen [Japanese patent application Nos. 2996/1984 and 47936/1985 (corresponding to U.S. Ser. No. 837,412)].
Furthermore, prior to the above-described proposal made by the applicants, a projector having a similar arrangement is known (Japanese Patent Unexamined Publication No. 109481/1982).
By utilizing the above-described proposal, it has become possible to provide a rear projection screen which enables the projector to be made compact by reducing the dimensions in depth and height and which provides sufficient brightness without reducing the resolving power.